As is widely known, diffractive optical elements that produce diffraction of light can be employed in a variety of applications. For example, wavelength multiplexers/demultiplexers, optical couplers, optical isolators, and like devices used in optical communications fields can be manufactured employing diffractive optical elements.
Diffractive optical elements generally are manufactured by forming a diffraction-grating layer on a transparent substrate. Diffractive optical elements are grossly divided, based on structural differences in the diffraction-grating layer, into modulated-refractive-index and surface-relief types.
FIG. 14 depicts, in a schematic sectional view, an example of a modulated-refractive-index type of diffractive optical element. It should be understood that in the drawings for the present application, dimensional proportions such as width and thickness have been altered as appropriate in order to clarify and simplify the figures, and do not reflect the proportions in their actual relationships. This modulated-refractive-index optical element includes a diffraction-grating layer 12a formed on a transparent substrate 11, wherein a modulated-refractive-index structure has been created in the diffraction-grating layer 12a. In particular, local regions having a relatively small refractive index n1 and local regions having a relatively large refractive index n2 are periodically formed in alternation in the diffraction-grating layer 12a. This enables the occurrence of a diffraction phenomenon originating in the phase difference that arises between light passing through the regions of low refractive index n1 and light passing through the regions of high refractive index n2.
The diffraction-grating layer 12a having the modulated-refractive-index structure can be formed utilizing for example a material whose refractive index is increased by the material undergoing energy-beam irradiation. For instance, increasing the refractive index of Ge-doped quartz glass by means of ultraviolet irradiation is known. Likewise, irradiating quartz glass with X-rays to increase the refractive index of the glass is known. Accordingly, a diffraction-grating layer 12a as illustrated in FIG. 14 can be created by depositing a quartz-glass layer of refractive index n1 onto a transparent substrate 11 and irradiating the glass layer with an energy beam in a periodic pattern to raise the refractive index locally to n2.
FIG. 15 illustrates, in a schematic sectional view, an example of a surface-relief type of diffractive optical element. This surface-relief diffractive optical element includes a diffraction-grating layer 12b formed on a transparent substrate 11, wherein a relief structure has been embossed in the diffraction-grating layer 12b. In particular, local regions having a relatively large thickness and local regions having a relatively small thickness are periodically formed in alternation in the diffraction-grating layer 12b. This enables the occurrence of a diffraction phenomenon originating in the phase difference that arises between light passing through the regions of large thickness and light passing through the regions of small thickness.
The diffraction-grating layer 12b having the surface-relief structure can be formed by for example depositing a quartz glass layer onto a transparent substrate 11 and employing photolithography and etching to process the glass layer.
FIG. 16 depicts, in a schematic sectional view, one more example of a modulated-refractive-index type of diffractive optical element. The modulated-refractive-index optical element of FIG. 16 resembles that of FIG. 14, but within a diffraction-grating layer 12c in FIG. 16 local regions having refractive indices n1, n2, n3 of three levels that differ from each other are arrayed periodically. Local regions in this way having three levels of refractive indices n1, n2, n3 can be formed within a diffraction-grating layer 12c by for example depositing onto a substrate 11 a quartz glass layer of refractive index n1 and irradiating the glass layer with an energy beam having two different energy levels.
By means of a diffraction grating that contains local regions whose refractive indices are multilevel, diffraction efficiency can be improved by comparison to the case with a diffraction grating that contains regions whose refractive indices are simply binary. “Diffraction efficiency” herein means the ratio of the sum of the diffracted light energies to the energy of the incident light. This means that from the perspective of putting diffracted light to use, greater diffraction efficiency is to be preferred.
FIG. 17 represents, in a schematic sectional view, one more example of a surface-relief type of diffractive optical element. The surface-relief optical element of FIG. 17 resembles that of FIG. 15, but within a diffraction-grating layer 12d in FIG. 17 local regions having thicknesses in three levels that differ from each other are arrayed periodically. Local regions in this way having refractive thicknesses in three levels can be formed within a diffraction-grating layer 12d by for example depositing onto a substrate 11 a quartz glass layer and repeating a photolithographic and etching process on the glass layer two times. Thus by means of a diffraction grating that contains local regions having a multilevel profile, diffraction efficiency can be improved by comparison to the case with a diffraction grating that contains simple binary thicknesses.
It should be noted that while modulated-refractive-index diffraction gratings in which the refractive indices within the diffraction grating layer are varied in stages are illustrated in FIGS. 14 and 16, also formable are modulated-refractive-index diffraction gratings in which the refractive indices are varied continuously. In that case the energy level of the energy beam used for irradiating in order to raise the refractive index should be varied continuously.
FIG. 18 schematically represents an example of the use of a diffractive optical element in an optical communications application. In the figure, collimators C0, C1 and C2 are respectively joined to the end faces of optical fibers F0, F1 and F2. Parallel-ray beam L, introduced through optical fiber F0 and output via collimator C0, can be split by a diffractive optical element DE into, for example, a beam of wavelength λ1 and a beam of wavelength λ2. This is because the diffraction angle of the beam will differ depending on the wavelength λ.
Thus a beam having a wavelength of λ1 can be input from collimator C1 into optical fiber F1, while a beam having a wavelength of λ2 can be input from collimator C2 into optical fiber F2. In other words, the demultiplexing functionality of the diffractive optical element DE is exploited in this case. Of course, conversely, a beam of wavelength λ1 output from optical fiber F1 via collimator C1, and a beam of wavelength λ2 output from optical fiber F2 via collimator C2 can be combined through the diffractive optical element DE and input into optical fiber F0 via collimator C0. This means that the diffractive optical element DE can demonstrate multiplexing/demultiplexing functionality. Thus a diffractive optical element of this sort having wavelength-division multiplexing/demultiplexing functionality is able to perform a crucial role in wavelength-division multiplexed (WDM) optical communications.
Although modulated-refractive-index diffractive optical elements such as described above are manufacturable in principle, in practice producing modulated-refractive-index diffractive optical elements is problematic. The reason is because the amount of refractive-index variation obtainable by irradiating for example quartz glass with an energy beam is at the very most 0.002 or so, which is prohibitive of creating an effective diffraction-grating layer.
Consequently, the general practice at present is—as set forth for example in Patent Reference 1, Japanese Unexamined Pat. App. Pub. No. S61-213802, and in Non-Patent Reference 1, Applied Optics, Vol. 41, 2002, pp. 3558–3566—to employ surface-relief types as diffractive optical elements. Nevertheless, the photolithography and etching necessary for fabricating relief diffractive optical elements are considerably complex manufacturing processes requiring a fair amount of time and trouble, besides which controlling the etching depth with precision is no easy matter. What is more, a problem with surface-relief diffractive optical elements is that since microscopic corrugations are formed in the element face, dust and dirt are liable to adhere.
Meanwhile, in a drop optical circuit such as represented in FIG. 18, the diffractive optical element DE, some several mm across, must be aligned and fixed in place with respect to the semiconductor laser LD and the optical fibers F0 through F2 atop a (non-illustrated) support base. This means that in a conventional diffractive optical element, the optical fibers are separate, individual optical components, which costs trouble in handling and has been prohibitive of scaling down the optical path.
An object of the present invention, in view of the situation as in the foregoing with prior technology, is efficiently and at low cost to make available optical fibers capped on an end with a working, tiny optically diffractive film.
Patent Reference 1
Japanese Unexamined Pat. App. Pub. No. S61-213802.
Non-Patent Reference 1
Applied Optics, Vol. 41, 2002, pp. 3558–3566.